Cerebrovascular regulation during heat stress.

Given that the brain is a highly metabolic organ and is enclosed by the skull, effective regulation of cerebral blood flow (CBF), not only for oxygen and nutrients supply but also as a means of convective heat exchange, is critical for preventing excessive elevations in brain temperature. Exposure to a hot environment can increase brain temperature while also decreasing CBF. This reduction in CBF may decrease heat removal from the brain and further increases its susceptibility to overheating. Therefore, understanding the impact of heat stress on CBF regulation, and its underlying mechanisms, is crucial for developing appropriate countermeasures and preventive strategies against heat-related injuries and mortality. Because hyperthermia profoundly complicates central circulatory regulation via extensive cutaneous vasodilation and blood volume distribution, CBF cannot be interpreted in isolation from systemic cardiovascular adjustments, particularly the preferential diversion of cardiac output toward the skin and extracranial tissues and the associated reduction in intracranial perfusion. Consequently, quantitative and region-specific assessments of cerebral perfusion are required, as commonly used velocity-based measurements may not fully capture changes in flow magnitude or differential responses between anterior and posterior cerebral circulations. Finally, since various brain functions are localized, it is essential to address regional differences in cerebral perfusion during stressors such as heat stress. The purpose of this review is to describe the key factors involved in the regulation of CBF and summarize how CBF is modulated under heat stress, with a focus on both quantitative changes and regional variability.